To resolder and hurt nothing. Disassemble everything including the valve internals. Clean the joint with a small brush and acetone. Lightly flux the joint and wrap some small diameter copper wire tightly around the barrel tube and breech to hold it all together when the solder melts. Set the oven temp. to 500 degrees and place the gun in the oven, lay it on a piece of aluminum foil. Let it get hot and occasionally touch a small piece of small diameter solder wire to the joint to see if the original solder is soft. If the solder is soft put some flux on a piece of small diameter copper wire and drag the wire the length of the joint. That will get the solder to flow, if it hasn't done it on it's own. Let cool off.
Here is the Sheridan front sight. Reminds me of an Orca in captivity. I will call this Blue Streak "Shamoo".